In the field of optical communications, optical fiber collimators are conventionally used to emit signal light, which has been transmitted through an optical fiber, in the form of parallel light from the optical fiber, or in reverse, used to converge parallel light on one end surface of an optical fiber to cause the light to enter the optical fiber. When using such an optical fiber collimeter, interposition of an optically functional element (for example, an optical filter, optical isolator, optical switch, optical modulator, etc.) between a pair of collimator lenses can provide a desired effect on the signal light transmitted through a single mode optical fiber at the light-entering side and then converge the signal light to further transmit the light to a single mode optical fiber at the light-receiving side.
Various forms of lenses have been used as collimating lenses for optical fiber collimators. However, cylindrical graded refractive index lenses (also referred to as “rod lenses” or “GRIN lenses”) are generally used because of their ease of polishing and like processing during manufacture, compared to spherical lens and complicated non-spherical lenses. Such graded refractive index lenses work as lenses, for example, converging light, because the interior of the rod glass has a continuous refractive index distribution in the radial direction from the center.
To produce such graded refractive index lenses, ion-exchange methods, double crucible methods, CVD methods (vapor-phase deposition methods), sol-gel methods, rod-in-tube methods, and like methods are known as techniques for forming a graded refractive index distribution in the radial direction of a glass rod. Among these, ion exchange is the most typical method for producing a graded refractive index lens and comprises immersing a homogeneous glass rod into a molten salt containing a monovalent cation (e.g., K+, Tl+, Ag+) to exchange a monovalent cation in the glass (e.g., Na+) for the monovalent cation in the molten salt, thereby forming a graded refractive index distribution. For example, Patent Document 1 discloses a method of producing a graded refractive index lens comprising subjecting a Na-containing glass rod to ion exchange using a molten salt containing Ag+ to form a graded reflective index distribution in the radial direction of the rod.
Microlens arrays comprising an array of lenses with a diameter of about several tens of micrometers to submillimeter, each lens having a graded refractive index distribution formed by subjecting a planer glass to ion exchange, are being used as computer board connectors or light source collimaters.
When a graded refractive index lens is produced by an ion exchange method, the temperature of the molten salt used is usually in the range of about 250° C. to about 400° C., and the manufacturing facility costs less than that of vapor-phase methods such as CVD. Moreover, compared to manufacturing spherical lenses, ion exchange methods are advantageous in terms of ease of polishing. However, conventional ion exchange methods have problems as below:
The first problem is the control of conditions of the molten salt at the time of ion exchange. The ion exchange rate and the rate of ion diffusion in a glass substrate depend on the temperature of the molten salt. The liquid phase temperature of the molten salt depends on the mixing ratio (composition) of the molten salt, and the ion exchange temperature can be controlled only at temperatures not lower than the liquid phase temperature of the molten salt. Therefore, there are cases in which the concentration of ions in the molten salt and the ion exchange temperature cannot be controlled independently. Therefore, to produce a graded refractive index lens having a desired refractive index profile by an ion exchange method, it is not easy to select the appropriate ion exchange conditions such as the molten salt composition, temperature, immersion time, etc., and a high level of expertise is needed. Furthermore, when using ions that are prone to oxidation in air, ion exchange needs to be performed in a reducing atmosphere. These problems are encountered not only in the production of graded refractive index lenses but are also common problems in the production of graded refractive index optical elements by introducing a graded refractive index distribution into a glass substrate.
The second problem is the application of an ion exchange-blocking film. When ion exchange is performed using a molten salt, it is necessary to apply an ion exchange-blocking film over the entire substrate except the portion where a graded refractive index distribution is to be formed. Photolithography is generally used to apply an ion exchange-blocking film, but formation of such a blocking film requires a complicated process. This is also a common problem in the production of graded refractive index optical elements by introducing a graded refractive index distribution into a glass substrate. Patent Document 1: Japanese Unexamined Patent Publication No. 2001-159702